Articles such as decals, labels, tapes, etc., which employ an adhesive-back printed or printable surface have been in general use for many years. The basic construction of a decal or sheet label comprises (1) a face, (2) a liner or backing material, and (3) a layer pressure-sensitive adhesive between the face and the liner. Because the surface of the liner which is in contact with the pressure-sensitive adhesive is coated with a release agent, the strength of the adhesive bond to the face is greater than to the liner. Thus, when the liner is peeled from the face, the pressure-sensitive adhesive remains attached to the back of the face. The face comprises at least one layer of sheet or film material, and the liner also comprises one or more layers of sheet or film material.
The portion of the article which remains after the liner has been removed from the pressure-sensitive adhesive backing is the desired pressure-sensitive adhesive-backed label, tape or decal which generally comprises a multi-layer face and the pressure-sensitive adhesive. The faces of labels may be made from various materials, and in some applications, it is desired to prepare the label from paper because of its low cost, strength and printability. In other applications, it is desired to use non-paper materials such as polymeric sheet materials which are often preferred because of their toughness, resistance to moisture and improved film characteristics. In some instances, prior art labels and the release liners used therewith have both been made from paper. Alternatively, the face is made from one material such as polyvinyl chloride and the liner is made from a different material such as paper, or the face is made of paper and the liner comprises a polymer film such as a polyvinyl film.
One of the advantages of pressure-sensitive adhesive-coated labels with release liners is that information can be printed on the label prior to attaching the label to another article such as a package, a shelf, etc. It is desirable that such information can be recorded on such labels utilizing laser printers or copying machines. Therefore, the labels and release liners must be able to withstand the conditions of the laser printers and copying machines such as elevated temperatures, pressures, etc. In many instances, attempts to print labels utilizing laser printers and copiers results in unacceptable curling of the label due at least in part to the high temperatures to which the label is subjected during the printing process. Curling of the label is undesirable because the curled labels may jam in the printer and/or the desired printed image will be fuzzy or uneven and unacceptable.
Various attempts have been made to eliminate or reduce curling in pressure-sensitive adhesive-backed labels by using various materials and combinations of materials in the face and release liners. Some of the attempts which have been made have been summarized in U.S. Pat. No. 5,154,956 (Fradrich) which describes a xerographically printable label. In particular, the label described in the '956 patent comprises a layer of sheet material, e.g., paper or polyvinyl chloride film having a printable upper surface and an adhesive-coated lower surface, and a release liner removably adhering to the lower surface of the layer of the sheet material. The release liner comprises a first layer of paper having a release coating on its upper surface, a water-impermeable adhesive on its lower surface, and a layer of textured paper (for example, embossed paper) bonded to the lower surface first layer of the water-impermeable adhesive. The presence of the textured paper is reported to provide the adhesive label with non-curling properties.
U.S. Pat. No. 5,154,956 also describes a prior attempt to eliminate or reduce curling which used release liners comprised of a sheet of paper sandwiched between two layers of polyethylene film with one of the polyethylene layers having a release agent coated on its outer surface. The patentee reports that such prior attempts have been unsuccessful because the polyethylene film-covered release liners have a tendency to acquire a static electric charge which prevents the labels from feeding smoothly into laser printers and copying machines.